The invention relates to a method for calibrating transmitters of ultrasonic flow meters according to the transit time difference method.
Ultrasonic flow meters are widely used in numerous industrial sectors. Ultrasonic flow meters according to the transit time difference method measure the difference of the transit time of two acoustic signals that propagate in and against the flow direction and use the transit time difference to calculate the volumetric flow rate. The transit time difference Δt depends on the average flow velocity VI along the sound path, the angle of incidence α in the fluid and the transit time tl of the acoustic signal in the fluid. The following equation shows the calculation of the flow velocity:VI=KT*(Δt/2tl)  (1)
KT is the transducer constant that determines the angle of incidence in the fluid. In case of acoustic transducers that are mounted on the outside of the tube, called clamp-on acoustic transducers, the following equation applies:KT=cα/sin(α)  (2)
The angle of incidence in the fluid is defined by the law of refraction and is derived from the angle of incidence α and the sound speed cα in the acoustic transducer. In order to calculate the volumetric flow rate, it is necessary to determine the fluidmechanical calibration factor KF, which shows the relationship between the average flow velocity across the cross-section area VA and the average flow velocity along the sound path VI:KF=VA/VI  (3)
Therefore, the volumetric flow rate Q is calculated as follows from the cross-section area A of the tube:Q=KF*A*KT*(Δt/2tl)  (4)
Usually, ultrasonic flow meters are calibrated with the help of a flow calibration rig. The flow meter is inserted in the pipe. In the case of clamp-on acoustic transducers, the acoustic transducers are instead mounted on the pipe. The displayed volumetric flow rate is compared with the reference volumetric flow rate and a calibration factor is calculated, if necessary. In the case of clamp-on transducers, while the measuring pipe in which the fluid flow being measured occurs is part of the measuring apparatus, usually it can not be calibrated together with the transmitter. Therefore, the calibration result can not be completely applied to the future measurement point. In order to avoid this disadvantage, it would be necessary to remove the pipe from the measurement point and install it in the calibration rig. Doing this would abandon the most important advantage of the clamp-on measurement, namely the non-intrusiveness. In some cases, it is only possible to calibrate the flow meter on site if a corresponding reference measurement is available at the facility. Mostly, this is not the case. Usually, a clamp-on measurement is calibrated by mounting the acoustic transducers onto the pipe of a calibration rig. Preferably, the pipe of the calibration rig has the same nominal diameter as the pipe on which the flow meter is to be installed later. The fluidmechanical conditions of the calibration rig, namely the flow profile and the pipe geometry, act as additional sources of error. Therefore, it is necessary to make sure that the flow profile is ideal. This can require a lot of effort, especially with large nominal diameters. Besides, it is necessary to determine the tube geometry with a very high accuracy. Therefore, it is preferable to create a calibration method that only contains parts of the measuring apparatus that will remain unchanged when used at the future measurement point.
In DE 102004031274 B4 a method is described that provides for the calibration of clamp-on acoustic transducers where the acoustic calibration factor KT of ultrasonic flow meters is calibrated without the need for a reference volumetric flow rate. A difference in position of the acoustic transducers is achieved by moving them with respect to each other and the associated transit time difference is measured. The emitting surfaces of the two acoustic transducers are arranged in parallel planes on acoustically opposite sides of the tube. It is not possible to calibrate the transmitters with this arrangement.
DE3029140A1 describes a device for the calibration of acoustic flow velocity meters by simulating transit times by means of delay elements. In order to be able to use this device for calibrating a transmitter, a modification of the circuits of the transmitter is required. Furthermore, the patent describes a delay line that contains electro-acoustic transducers. With this method, it is possible to calibrate the entire meter, including the acoustic transducers. However, it is not possible to calibrate the transmitter independently of the acoustic transducers.
U.S. Pat. No. 4,762,012A describes an arrangement that provides for the testing of the transmitter without using the acoustic component of the measuring apparatus. The acoustic transducers and the other elements of the measuring apparatus, such as the pipe and the flowing fluid, are simulated electronically. For this, two delay generators are proposed. The first delay generator, started by the emitted signals of the transmitter, simulates the transit time of the stationary fluid. The second delay generator, triggered by the first one, generates an additional delay which, when operation in and against the flow direction is simulated, is different and therefore simulates the influence of the flow direction on the transit time of the acoustic signals. The second delay generator starts a signal shaping network, which generates the artificial signals fed into the transmitter. The invention provides for the simulation of discrete transit times with a quantization that depends on the clock rate of the delay generators. The simulated signal shape can not be selected freely but is defined by the signal shaping network.
US2009000392A1-1 also contains a description of a measuring apparatus where the transit times of the signals in and against the flow direction are generated by means of a delay generator. A rough delay is combined with a fine delay. The signals are generated by a digital-to-analog converter, which converts a digital representation of the signals to be simulated to analog signals. The digital-to-analog converter is started by the delay generator.